The invention relates generally to field regulation of wound field synchronous machines and more particularly to off-highway vehicle alternator controls.
Self-propelled traction vehicles such as large off-highway haulage trucks include electric propulsion systems. A typical propulsion system, such as described in commonly assigned Black et al., U.S. Pat. No. 4,495,449, includes an operator-controlled throttle adapted to control the rotational speed of a prime mover which may, for example, comprise a diesel engine. An output shaft of the prime mover is drivingly coupled to a rotor of an alternating current (AC) generator (a wound field synchronous machine commonly referred to as an alternator) which has a set of three-phase main windings, an auxiliary (tertiary) winding, and a field winding. The three-phase, generally sinusoidal, alternating voltage that is generated in the main windings of the alternator is converted to direct voltage by means of an uncontrolled full-wave rectifying bridge whose output in turn is coupled either (a) to at least one armature of a respective adjustable speed direct current (DC) traction motor or (b) through an inverter to a respective alternating current (AC) traction motor. The motor rotor is coupled through suitable speed-reduction gearing to a pair of wheels located on opposite sides of the vehicle. For an AC traction system, by controlling the speed of the engine, the excitation of the alternator, and the inverter torque commands, the vehicle can be propelled (also known as “motoring”) or dynamically retarded (also known as “electric braking”) by the vehicle's motor or motors in either a forward or a reverse direction.
For DC traction motors, during the motoring mode of operation, the motor will rotate at a speed that depends on both the magnitude of excitation current in the motor field and the magnitude of the voltage applied to the armature windings. For AC traction motors, a more complex voltage control system is typically implemented through one set of armature windings to control field excitation and torque producing armature current.
The magnitude of the voltage applied to the armature windings is a function of both the speed at which the alternator is driven and the magnitude of excitation current in the alternator field. The alternator field excitation current is supplied by the field winding of the alternator via a single-phase, full-wave “phase controlled” rectifying bridge. Alternator field excitation current magnitude depends on the timing of periodic firing signals that are supplied to the rectifier from a firing angle control of a controller.
Present implementations for the regulation and control of the field and output voltage in wound rotor synchronous alternators are subject to parameter and signal variations. Typically the rectifier that supplies DC current to the alternator field winding comprises a thyristor such as a silicon controlled rectifier (SCR) bridge, for example. Thyristor based (or other phase control based) rectification experiences inherent non-linear behavior, and thus control presents several control challenges.
An exemplary description of rectifier circuits is provided in JOHANNES SCHAEFER, RECTIFIER CIRCUITS: THEORY AND DESIGN 1-126 (John Wiley & Sons, Inc. 1965). Conventional phase-controlled rectifier systems include techniques based on analog circuitry wherein AC voltage is rectified to form DC voltage that is applied to the alternator field winding. The average value of the DC voltage is modulated or controlled by varying the firing angle of the rectifier bridge. To accomplish the modulation, a ramp waveform that is synchronous in phase and frequency to the rectified AC voltage is compared to a small signal reference command signal. The crossing of the two signals establishes the timing of the turn-on commands that switch the rectifier bridge.
The analog circuitry of conventional techniques is inherently inflexible to modifications in that any design changes require hardware changes. Additionally, the gain of the circuitry is non-linear and highly sensitive to the operating point of the firing angle, the speed of the alternator, the level of field excitation, and the load variation. The gain is additionally sensitive to other variations in the AC voltage such as distortion due to temperature induced variation. The speed and field excitation level both change the amplitude of the AC voltage which leads directly to changes in the amount of voltage applied given a certain firing angle. In addition, the small signal gain from the reference to the field voltage is based on a time-averaged value of the discrete pulses of field voltage. This relation imposes limitations on the outer control loops in that the bandwidths must be significantly lower (typically on the order of about ten) than the pulse frequency. The inherent non-linear transfer function of the time-averaged value imposes further constraints on the outer control loops. The outer control loops must be stabilized for all operating points, which means that performance will be compromised to ensure stability at the worst case operating points. For example, the outer control loop gains and bandwidths are often set to be sufficiently low so as to accommodate the least stable operating points of the rectifier bridge to ensure overall stability.
It would therefore be desirable to have a wound field synchronous machine control system that is robust to parameter and operating point variations, insensitive to non-linearities, and readily adaptable to design modifications.